(a) Field of the Invention
The present invention relates to an image-transmitting optical system used for a device such as an endoscope (which is not flexible), especially an image-transmitting optical system using inhomogeneous lenses.
(b) Description of the Prior Art
Conventional image-transmitting optical system used for endoscopes, etc., comprised only homogeneous lenses. That is, an optical system for an endoscope, as shown in FIG. 1, comprises an objective lens 1, an image-transmitting optical system 2 and an eyepiece 3. Among them, the image-transmitting optical system 2 may be, for example, of a lens as shown in FIG. 2 that is disclosed in Japanese Published Examined patent application No. 5993/74. In this optical system, curvature of field is not eliminated.
There exists prior art which has made an improvement and that is a lens system disclosed in Japanese Published Unexamined patent application No. 4245/77, as shown in FIG. 3. In this lens system, curvature of field is substantially eliminated. This lens system, however, has defects in that, for example, the lens construction is complex, the number of lenses is large, and there are many air-contact surfaces.
Recently, graded refractive index lenses (inhomogeneous lenses) have been developed.
An optical system for an endoscope using the inhomogeneous lens is shown in FIG. 4. In this optical system for an endoscope, there is one image-transmitting optical system 4 comprising an inhomogeneous lens both surfaces of which are plane. This inhomogeneous lens of the image-transmitting optical system has the refractive index whose distribution decreases substantially with the square of the distance from the center to a periphery of the lens so that light ray travels in a curve, as shown in FIG. 5, and an image is transmitted. In the image-transmitting optical system using this inhomogeneous lens, the construction is very simple but curvature of field is not eliminated.